Positive electrode for secondary batteries, and secondary battery

Abstract

This positive electrode for secondary batteries is provided with a positive electrode collector 11 and a positive electrode mixture layer that is arranged on a surface of the positive electrode collector; and the positive electrode mixture layer comprises a first layer 12A that is in contact with the positive electrode collector and a second layer 12B that is in contact with the first layer. The first layer 12A contains first positive electrode active material particles that have a particle diameter L. The second layer 12B contains second positive electrode active material particles that have a particle diameter R. Third positive electrode active material particles that have a particle diameter r are contained at least in the interface between the first layer and the second layer. The first positive electrode active material particles contain a lithium transition metal composite oxide in which the proportion of Co in the metal elements other than Li is 2% by atom or more. The second and third positive electrode active material particles contain a lithium transition metal composite oxide in which Co is not contained or alternatively, the proportion of Co in the metal elements other than Li is less than 2% by atom. The particle diameters of the first to third positive electrode active material particles satisfy the relational expression R > L > r.

Description

Positive electrode for secondary battery and secondary battery

[0001] The present disclosure relates to a positive electrode for a secondary battery and a secondary battery.

[0002] BACKGROUND ART Secondary batteries, especially lithium-ion secondary batteries, have high output and high energy density, and are therefore expected to be used as power sources for small consumer applications, power storage devices, and electric vehicles.

[0003] Patent Document 1 discloses a positive electrode for a lithium secondary battery that includes a positive electrode mixture layer containing a lithium transition metal composite oxide as a positive electrode active material and a current collecting foil, in which the positive electrode mixture layer is made of α-NaFeO ２ and a layer in contact with the current collecting foil and containing a lithium transition metal composite oxide in which the transition metal (Me) contains one or more selected from Co, Ni and Mn, the molar ratio of lithium (Li) to the transition metal (Li / Me) is 1.2 or less, and the molar ratio Mn / Me is 0.5 or more.

[0004] JP 2015-115244 A

[0005] In recent years, from the viewpoint of reducing production costs, there has been a demand for the use of lithium transition metal composite oxides that have a low cobalt (Co) content or that do not contain cobalt as the positive electrode active material. Among these, lithium-nickel-manganese composite oxides containing Ni and Mn, such as LiNi １－ｘ Mn ｘ O ２ (x≦0.2) is considered promising due to its large charge / discharge capacity.

[0006] However, the above-mentioned lithium-nickel-manganese composite oxide has lower electronic conductivity than conventionally used lithium transition metal composite oxides containing cobalt (e.g., lithium-nickel-cobalt-aluminum composite oxide). Furthermore, the presence of an impurity layer on the surface reduces current collection. In addition, the contact resistance with the aluminum foil used as the current collector is also high. For these reasons, lithium-nickel-manganese composite oxide has high resistance and is prone to large polarization. Furthermore, as charge-discharge cycles are repeated, gaps form between the active material and the aluminum foil due to expansion and contraction, which tends to increase resistance. As a result, secondary batteries using lithium-nickel-manganese composite oxide as the positive electrode active material are prone to poor charge-discharge characteristics.

[0007] One aspect of the present disclosure provides a positive electrode current collector and a positive electrode mixture layer provided on a surface of the positive electrode current collector, the positive electrode current collector including Al, the positive electrode mixture layer including a first layer in contact with the positive electrode current collector and a second layer in contact with the first layer, the first layer including first positive electrode active material particles having a particle size L, the second layer including second positive electrode active material particles having a particle size R, and third positive electrode active material particles having a particle size r at least at an interface between the first layer and the second layer, the first positive electrode active material particles including a first lithium transition metal composite oxide, and Co accounting for metal elements other than Li contained in the first lithium transition metal composite oxide. the second positive electrode active material particles contain a second lithium transition metal composite oxide, and the second lithium transition metal composite oxide does not contain Co or the proportion of Co in the metal elements other than Li contained in the second lithium transition metal composite oxide is less than 2 atomic %; the third positive electrode active material particles contain a third lithium transition metal composite oxide, and the third lithium transition metal composite oxide does not contain Co or the proportion of Co in the metal elements other than Li contained in the third lithium transition metal composite oxide is less than 2 atomic %, and R>L>r is satisfied.

[0008] Another aspect of the present disclosure relates to a secondary battery including the above-described positive electrode for secondary batteries, a separator, a negative electrode facing the positive electrode with the separator interposed therebetween, and an electrolyte solution.

[0009] According to the present disclosure, a secondary battery having a high capacity and advantageously improved charge / discharge characteristics can be realized.

[0010] The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0011] FIG. 1 is a cross-sectional view schematically illustrating the structure of a positive electrode according to an embodiment of the present disclosure. FIG. 2 is a schematic perspective view, with a portion cut away, of a secondary battery according to an embodiment of the present disclosure. FIG. 3 is a graph illustrating charge / discharge curves of a battery of Example 1. FIG. 4 is a graph illustrating charge / discharge curves of a battery of Comparative Example 1. FIG. 5 is a graph illustrating changes in capacity retention rates per charge / discharge cycle for the batteries of Example 1 and Comparative Examples 1 and 2 under conditions of charging to an overcharged state. FIG. 6 is a graph illustrating charge / discharge curves of a battery of Example 6.

[0012] Hereinafter, embodiments of the present disclosure will be described using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values, materials, etc. may be exemplified, but other numerical values, materials, etc. may be applied as long as the effects of the present disclosure are obtained. In this specification, the expression "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or more and numerical value B or less." In the following description, when lower and upper limits of numerical values ​​related to specific physical properties or conditions are exemplified, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not equal to or greater than the upper limit. When multiple materials are exemplified, one of the materials may be selected and used alone, or two or more of the materials may be used in combination.

[0013] The present disclosure also encompasses combinations of two or more features arbitrarily selected from the appended claims, i.e., two or more features arbitrarily selected from the appended claims can be combined unless a technical contradiction arises.

[0014] A positive electrode for a secondary battery according to an embodiment of the present disclosure includes a positive electrode current collector and a positive electrode mixture layer provided on the surface of the positive electrode current collector. The positive electrode current collector is made of a sheet-like conductive material containing Al. The positive electrode current collector is, for example, an aluminum foil or an aluminum alloy foil. The positive electrode mixture layer is supported on one or both surfaces of the positive electrode current collector. The positive electrode mixture layer is typically a layer (including a membrane or film) made of a positive electrode mixture. The positive electrode mixture contains a positive electrode active material as an essential component.

[0015] The positive electrode mixture layer includes a first layer in contact with the positive electrode current collector and a second layer in contact with the first layer. In other words, the first layer and the second layer are stacked in this order on the positive electrode current collector, with the first layer sandwiched between the positive electrode current collector and the second layer. The first layer contains first positive electrode active material particles with a particle size L. The second layer contains second positive electrode active material particles with a particle size R. Furthermore, third positive electrode active material particles with a particle size r are present at least at the interface between the first layer and the second layer. The third positive electrode active material particles may be contained throughout the second layer.

[0016] The particle size L of the first positive electrode active material particles, the particle size R of the second positive electrode active material particles, and the particle size r of the third positive electrode active material particles satisfy the relationship R > L > r, where the particle sizes L, R, and r are each measured from a cross section in the thickness direction obtained by simultaneously cutting the positive electrode mixture layer and the positive electrode current collector, as described below.

[0017] The first positive electrode active material particles contain a first lithium transition metal composite oxide. The proportion of Co among the metal elements other than Li contained in the first lithium transition metal composite oxide is 2 atomic % or more. The first lithium transition metal composite oxide preferably contains Ni, Co, and Al. Such a first lithium transition metal composite oxide has high electronic conductivity and excellent adhesion to the aluminum foil serving as the positive electrode current collector. Therefore, by having the first layer containing the first lithium transition metal composite oxide in contact with the positive electrode current collector, current collection performance is improved and the resistance between the first layer and the positive electrode current collector can be reduced. This reduces polarization during rapid charging and improves charge / discharge characteristics.

[0018] Meanwhile, the first layer is also in contact with a second layer containing second positive electrode active material particles on the opposite side of the positive electrode current collector. The second layer contains a second lithium transition metal composite oxide. The second lithium transition metal composite oxide does not contain Co, or the proportion of Co among the metal elements other than Li contained in the second lithium transition metal composite oxide is less than 2 atomic %. A layer containing such a second lithium transition metal composite oxide has high resistance when in direct contact with the aluminum foil positive electrode current collector. However, in the positive electrode for secondary batteries according to this embodiment, the second layer containing the second lithium transition metal composite oxide is not in direct contact with the positive electrode current collector but is in contact with the first layer. Furthermore, the resistance between the first layer and the second layer is low, polarization is suppressed, and charge / discharge characteristics are improved.

[0019] At the interface between the first and second layers, at least a portion of the first positive electrode active material particles fits into the gaps (recesses) between the second positive electrode active material particles and contacts the second positive electrode active material particles in a manner that destroys the impurity layer on the surface of the second positive electrode active material particles. This improves the current collection properties of the second positive electrode active material particles. Here, it is believed that the particle size R of the second positive electrode active material particles is larger than the particle size L of the first positive electrode active material particles (R>L), which facilitates the formation of thick conductive paths and reduces the resistance between the first and second layers.

[0020] In addition, the third positive electrode active material particles having a particle size r smaller than the particle size L of the first positive electrode active material particles and the particle size of the second positive electrode active material particles R can be arranged at the interface between the first layer and the second layer so as to fill the gaps between the first positive electrode active material particles and the second positive electrode active material particles, thereby further improving the adhesion between the first layer and the second layer, further reducing the resistance between the first layer and the second layer, and further improving the charge / discharge characteristics.

[0021] The third positive electrode active material particles contain a third lithium transition metal composite oxide. Like the second lithium transition metal oxide, the third lithium transition metal composite oxide does not contain Co, or the proportion of Co to the metal elements other than Li contained in the third lithium transition metal composite oxide is less than 2 atomic %. The third lithium transition metal composite oxide may be a composite oxide having the same composition as the second lithium transition metal oxide except for the particle size, or may be a composite oxide different from the second lithium transition metal oxide.

[0022] The third positive electrode active material particles may be contained inside the second layer, in addition to at the interface between the first layer and the second layer. In this case, the third positive electrode active material particles may be arranged in the second layer so as to fill gaps between the second positive electrode active material particles. This increases the filling rate of the positive electrode active material particles in the positive electrode mixture layer, resulting in a high capacity, and further reduces the resistance of the second layer, thereby further improving charge / discharge characteristics.

[0023] On the other hand, the third positive electrode active material particles may not be contained within the first layer, and the third positive electrode active material particles may not be present at the interface between the positive electrode current collector and the first layer. In this case, the third positive electrode active material particles do not fill the gaps between the first positive electrode active material particles in the first layer, and the electrolyte can be held in the gaps between the first positive electrode active material particles. This makes it possible to maintain a high capacity even during rapid charge / discharge and improve rate characteristics.

[0024] The absence of third positive electrode active material particles at the interface between the positive electrode current collector and the first layer means that, when observing a cross section of the positive electrode, there are no third positive electrode active material particles, or at most three third positive electrode active material particles, in the space formed between the surface of the positive electrode current collector, one first positive electrode active material particle in contact with it, and the adjacent first positive electrode active material particle (this particle is also in contact with the positive electrode current collector).

[0025] The particle size r of the third positive electrode active material particles and the particle size L of the first positive electrode active material particles preferably satisfy the relationship r > 0.155L. In this case, since r / L > (2√3 - 3) / 3 (= 1.1547), the third positive electrode active material particles at the interface between the first and second layers are prevented from penetrating into the first layer through gaps between the first positive electrode active material particles. Therefore, spaces capable of holding an electrolyte are formed in the gaps between the first positive electrode active material particles in the first layer, thereby improving the rate characteristics.

[0026] The particle size L of the first positive electrode active material particles, the particle size R of the second positive electrode active material particles, and the particle size r of the third positive electrode active material particles are each the maximum diameter obtained by observing a cross section in the thickness direction obtained by simultaneously cutting the positive electrode mixture layer and the positive electrode current collector and performing image processing. The cross section may be formed using a cross-section polisher (CP). In this case, a thermosetting resin may be filled into the positive electrode mixture layer and cured. Here, if the particles are not circular, the diameter (circle-equivalent diameter) of a circle having the same area as the cross section area of ​​the particle (the area of ​​the particle observed in the cross section of the positive electrode mixture layer) is obtained, and the maximum value of the circle-equivalent diameter is defined as the maximum diameter. Ten or more particles are observed to determine the maximum diameter.

[0027] When the third positive electrode active material particles are dispersed throughout the second layer, a cross section of the second layer in the thickness direction may contain a mixture of both the second positive electrode active material particles and the third positive electrode active material particles. When the second positive electrode active material particles and the third positive electrode active material particles can be distinguished from the image of the cross section, the maximum diameters of the second positive electrode active material particles and the third positive electrode active material particles can be determined, and R and r can be calculated. Furthermore, the distribution of the equivalent circle diameters of the positive electrode active material particles obtained by image processing may contain two peaks: one due to the second positive electrode active material particles and the other due to the third positive electrode active material particles. The two peaks in the distribution of equivalent circle diameters may be separated, and the maximum values ​​of the peaks due to the separated second positive electrode active material particles and the separated third positive electrode active material particles may be calculated, thereby obtaining R and r.

[0028] When the first positive electrode active material particles, the second positive electrode active material particles, and the third positive electrode active material particles can be separated and recovered from the positive electrode mixture layer, the D80 diameters (particle diameters at 80% cumulative volume) in the volume-based particle size distribution of each of the first positive electrode active material particles, the second positive electrode active material particles, and the third positive electrode active material particles may be determined as L, R, and r. The volume-based particle size distribution can be measured by a laser diffraction scattering method. For example, an "LA-750" manufactured by HORIBA, Ltd. can be used as the measuring device.

[0029] When the third positive electrode active material particles are dispersed throughout the second layer, the particle size distribution obtained by separating and recovering the positive electrode active material particles from the second layer will have two peaks: one due to the second positive electrode active material particles and the other due to the third positive electrode active material particles. In this case, the peaks due to the second positive electrode active material particles and the peaks due to the third positive electrode active material particles may be separated from the particle size distribution, and the D80 diameter may be determined from each peak in the particle size distribution, thereby determining R and r.

[0030] The particle size R of the second positive electrode active material particles may be, for example, in the range of 10 to 30 μm, or in the range of 10 to 25 μm, while the particle size r of the third positive electrode active material particles may be, for example, in the range of 1 to 5 μm.

[0031] Examples of the first lithium transition metal composite oxide constituting the first positive electrode active material particles include a lithium-nickel-cobalt-manganese composite oxide containing Ni, Co, and Mn (hereinafter referred to as "composite oxide NCM"), and a lithium-nickel-cobalt-aluminum composite oxide containing Ni, Co, and Al (hereinafter referred to as "composite oxide NCA"). Examples of the composite oxide NCM include LiNi ０．５ Co ０．２ Mn ０．３ O ２ , LiNi １／３ Co １／３ Mn １／３ O ２ The composite oxide NCA is Li α Ni １－ｘ－ｙ Co ｘ Al ｙ O ２(where 0.95≦α≦1.05, 0.02≦x≦0.1, 0.02<x+y<1) In the above formula, the α value indicating the molar ratio of lithium is the value when the positive electrode is discharged until the potential becomes 2.5 V versus the Li counter electrode, and it increases or decreases with charge and discharge.

[0032] The second lithium transition metal composite oxide constituting the second positive electrode active material particles and the third lithium transition metal composite oxide constituting the third positive electrode active material particles may be a lithium-nickel-manganese composite oxide containing Ni and Mn (hereinafter referred to as "composite oxide NM"). In the second lithium transition metal composite oxide or the third lithium transition metal composite oxide, the proportion of Ni and Mn in the metal elements other than Li contained in the composite oxide NM may be 98 atomic % or more. The composite oxide NM may contain Li, α Ni １－ｘ Mn ｘ O ２ (where 0.95≦α≦1.05, 0<x≦0.2) may also be used.

[0033] FIG. 1 is a cross-sectional view schematically illustrating the structure of a positive electrode for a secondary battery according to this embodiment. The positive electrode 10 includes a positive electrode current collector 11 and a positive electrode mixture layer 12 provided on the surface of the positive electrode current collector 11. FIG. 1 illustrates a portion of a cross section in the thickness direction obtained by simultaneously cutting the positive electrode mixture layer 12 and the positive electrode current collector 11. The positive electrode mixture layer 12 can be formed on both main surfaces of the positive electrode current collector 11. FIG. 1 illustrates a portion of one main surface of the positive electrode current collector 11 and a portion of the positive electrode mixture layer 12 formed on that main surface, and does not illustrate the other main surface of the positive electrode current collector 11.

[0034] The positive electrode mixture layer 12 includes a first layer 12A in contact with the positive electrode current collector 11 and a second layer 12B in contact with the first layer 12A on the side facing the positive electrode current collector 11 with the first layer interposed therebetween. The first layer 12A includes first positive electrode active material particles P1. The second layer 12B includes second positive electrode active material particles P2 and third positive electrode active material particles P3. The particle size L of the first positive electrode active material particles P1 is smaller than the particle size R of the second positive electrode active material particles P2 and larger than the particle size of the third positive electrode active material P3 (R > L > r).

[0035] The positive electrode current collector 11 is an aluminum foil. The first positive electrode active material particles P1 are a lithium-nickel-cobalt-aluminum composite oxide (composite oxide NCA) in this example, and the first positive electrode active material particles P1 are in contact with the positive electrode current collector 11 such that a portion of the first positive electrode active material particles P1 is embedded in the aluminum foil that is the positive electrode current collector 11. This brings the first positive electrode active material particles P1 into surface contact with the positive electrode current collector 11, reducing the resistance between the positive electrode current collector 11 and the first layer 12A.

[0036] The third positive electrode active material particles P3 are present in the gaps between the first and second positive electrode active material particles P1 and P2 at the interface between the first and second layers, and in the gaps between the second positive electrode active material particles P2 in the second layer, so that the second positive electrode active material particles P2 are in direct contact with the first positive electrode active material particles P1 and are in contact with the first positive electrode active material particles P1 via the third positive electrode active material particles P3.

[0037] On the other hand, the third positive electrode active material particles P3 are not present in the gaps between the first positive electrode active material particles P1 in the first layer. The gaps between the first positive electrode active material particles P1 are filled with the electrolyte solution. This improves the rate characteristics.

[0038] If the first layer is formed of spherical first positive electrode active material particles P1 with a diameter L and a close-packed structure, the diameter of the largest sphere that can exist in the gaps formed by the three first positive electrode active material particles P1 that form a triangular lattice within the plane is (2√3-3)L / 3 (=0.1547L). Therefore, if r>0.155L, the third positive electrode active material particles P3 present at the interface between the first and second layers are prevented from migrating toward the positive electrode current collector through the gaps between the first positive electrode active material particles P1. Therefore, in this case, the electrolyte can be retained in the gaps between the first positive electrode active material particles P1, improving the rate characteristics.

[0039] The second positive electrode active material particles P2 and the third positive electrode active material particles P3 are lithium-nickel-manganese composite oxide (composite oxide NM) in this example. By mixing the second positive electrode active material particles P2 with larger particle sizes and the third positive electrode active material particles P3 with smaller particle sizes in the second layer, the packing density of the positive electrode active material in the second layer can be increased, and the capacity can be increased.

[0040] Although FIG. 1 shows the positive electrode mixture layer 12 having a two-layer structure of the first layer 12A and the second layer 12B, another positive electrode active material layer may be present on the second layer 12B.

[0041] Next, a secondary battery according to an embodiment of the present disclosure will be described in detail. The secondary battery includes, for example, the positive electrode, a separator, a negative electrode facing the positive electrode with the separator interposed therebetween, and an electrolyte solution.

[0042] [Positive Electrode] The positive electrode comprises a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector and containing a positive electrode active material. The positive electrode active material layer contains the positive electrode active material as an essential component and may contain optional components such as a binder and a conductive agent. Known materials can be used as the binder, conductive agent, and thickener.

[0043] As described above, the positive electrode mixture layer has a laminated structure of at least two layers: a first layer in contact with the positive electrode current collector and a second layer in contact with the first layer. The thickness T1 of the first layer is, for example, 3 to 30 μm. The thickness T2 of the second layer is, for example, 10 to 150 μm. The ratio of the thickness T1 of the first layer to the thickness T2 of the second layer (T1 / T2) is, for example, 0.1 to 1.0.

[0044] As described above, the first layer contains first positive electrode active material particles having a particle size L. The second layer contains second positive electrode active material particles having a particle size R. Third positive electrode active material particles having a particle size r are present at least at the interface between the first layer and the second layer. The third positive electrode active material particles may be dispersed and arranged in the second layer together with the second positive electrode active material particles.

[0045] The first layer can be formed, for example, by a method including a process of applying a first positive electrode slurry, in which a first positive electrode mixture containing first positive electrode active material particles, a binder, etc. is dispersed in a dispersion medium, to the surface of a positive electrode current collector. The second layer can be formed, for example, by a method including a process of applying a second positive electrode slurry, in which a second positive electrode mixture containing second positive electrode active material particles, third positive electrode active material particles, and a binder, etc. is dispersed in a dispersion medium, to the surface of the first positive electrode slurry. The laminated coating film after drying may be rolled as necessary. The first positive electrode slurry and the second positive electrode slurry may be simultaneously applied to the surface of the positive electrode current collector using a two-fluid nozzle.

[0046] When the second layer contains second positive electrode active material particles and third positive electrode active material particles, the proportion of the second positive electrode active material particles in the total of the second positive electrode active material particles and the third positive electrode active material particles may be 50% to 90% or 65% to 85% by mass.

[0047] The first to third positive electrode active material particles may be lithium transition metal composite oxides containing lithium and Ni and having a layered rock salt crystal structure, although the metal elements other than lithium and the ratios of the metal elements in the lithium transition metal composite oxides may be varied in each of the first to third positive electrode active material particles.

[0048] Lithium transition metal composite oxides containing nickel are advantageous in achieving high capacity and low cost. From the viewpoint of obtaining high capacity, it is desirable that the proportion of Ni in the metal elements other than Li contained in the lithium transition metal composite oxide is 80 atomic % or more. The proportion of Ni in the metal elements other than Li may be 85 atomic % or more, or even 90 atomic % or more. The proportion of Ni in the metal elements other than Li is desirably, for example, 95 atomic % or less. When limiting the range, these upper and lower limits can be combined arbitrarily.

[0049] The lithium transition metal composite oxide may contain Co, Mn, and / or Al. Co, Mn, and Al contribute to stabilizing the crystal structure of a composite oxide with a high Ni content. However, from the viewpoint of reducing production costs, a lower Co content is preferable. A composite oxide with a low Co content or no Co may contain Mn and Al. From the viewpoint of reducing production costs, it is desirable that the proportion of Co in the metal elements other than Li in the lithium transition metal composite oxide be kept below 2 atomic %.

[0050] The lithium transition metal composite oxide is, for example, a compound represented by the general formula: Li α Ni1-x1-x2-y-zCo ｘ１ Mn ｘ２ Al ｙ Me ｚ O ２＋β However, the general formula satisfies 0.95≦α≦1.05, 0≦x1≦0.1, 0≦x2≦0.5, 0≦y≦0.1, 0≦z≦0.1, 0.5≦1-x1-x2-y-z, and -0.05≦β≦0.05, and Me is an element other than Li, Ni, Mn, Al, Co, and oxygen. The α value, which indicates the molar ratio of lithium, is the value when the positive electrode potential is charged to 2.5 V relative to the Li counter electrode, and increases or decreases with charge and discharge.

[0051] As Me, from the viewpoint of stabilizing the crystal structure of the composite oxide N, at least one selected from the group consisting of Nb, Zr, B, Mg, Fe, Cu, Zn, Sn, Na, K, Ba, Sr, Ca, W, Mo, Si, Ti, Fe, and Cr can be used.

[0052] Among the lithium transition metal composite oxides, the first lithium transition metal composite oxide used for the first positive electrode active material particles is Li α Ni １－ｘ－ｙ Co ｘ Al ｙ O ２(where 0.02≦x≦0.1, 0.02<x+y<1) can be used. Although the composite oxide NCA contains a relatively large amount of Co, it has high electronic conductivity and good adhesion to the positive electrode current collector, thereby reducing contact resistance. On the other hand, by reducing the ratio of the thickness of the first layer to the thickness of the positive electrode mixture layer, the increase in manufacturing costs due to the inclusion of Co can be minimized.

[0053] On the other hand, for the second and third positive electrode active material particles, the proportion of Co in the metal elements other than Li can be kept to less than 2 atomic % to reduce the manufacturing cost. Among the above lithium transition metal composite oxides, the second lithium transition metal composite oxide used for the second positive electrode active material particles and the third lithium transition metal composite oxide used for the third positive electrode active material particles are preferably Li α Ni １－ｘ Mn ｘ (where 0<x≦0.2) can be used. By forming the positive electrode mixture layer into two layers, a first layer containing the composite oxide NCA and a second layer containing the composite oxide NM, it is possible to achieve a high capacity while suppressing an increase in the resistance of the positive electrode.

[0054] The shape and thickness of the positive electrode current collector may be, for example, 5 μm or more and 20 μm or less. Examples of the material of the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0055] [Negative Electrode] The negative electrode includes, for example, a negative electrode current collector and a negative electrode active material layer formed on the surface of the negative electrode current collector. The negative electrode active material layer can be formed, for example, by applying a negative electrode slurry, in which a negative electrode mixture containing a negative electrode active material, a binder, etc. is dispersed in a dispersion medium, to the surface of the negative electrode current collector and drying the slurry. The dried coating may be rolled as necessary. In other words, the negative electrode active material may be a negative electrode mixture layer. Alternatively, a lithium metal foil or a lithium alloy foil may be attached to the negative electrode current collector as the negative electrode active material layer. The negative electrode active material layer may be formed on one surface or both surfaces of the negative electrode current collector.

[0056] The negative electrode active material layer contains a negative electrode active material as an essential component, and may contain optional components such as a binder, a conductive agent, a thickener, etc. Known materials can be used as the binder, conductive agent, and thickener.

[0057] Negative electrode active materials include materials that electrochemically absorb and release lithium ions, lithium metal, lithium alloys, etc. Materials that electrochemically absorb and release lithium ions include carbon materials and alloy-based materials. Examples of carbon materials include graphite, easily graphitized carbon (soft carbon), and hardly graphitized carbon (hard carbon). Of these, graphite is preferred because of its excellent charge / discharge stability and low irreversible capacity. Examples of alloy-based materials include those containing at least one metal that can form an alloy with lithium, such as silicon, tin, silicon alloys, tin alloys, and silicon compounds. Silicon oxides and tin oxides formed by bonding these with oxygen may also be used.

[0058] Examples of alloy-based materials containing silicon include a lithium ion conductive phase and a silicon composite material in which silicon particles are dispersed in the lithium ion conductive phase. Examples of lithium ion conductive phases that can be used include a silicon oxide phase, a silicate phase, and a carbon phase. The silicon oxide phase may be primarily composed of silicon dioxide (e.g., 95 to 100% by mass). Among these, composite materials composed of a silicate phase and silicon particles dispersed in the silicate phase are preferred because of their high capacity and low irreversible capacity. Furthermore, a silicate phase containing lithium (hereinafter also referred to as a lithium silicate phase) is preferred because of its low irreversible capacity and high initial charge / discharge efficiency.

[0059] The lithium silicate phase may be an oxide phase containing lithium (Li), silicon (Si), and oxygen (O), and may contain other elements. The atomic ratio of O to Si in the lithium silicate phase, O / Si, is, for example, greater than 2 and less than 4. Preferably, O / Si is greater than 2 and less than 3. The atomic ratio of Li to Si in the lithium silicate phase, Li / Si, is, for example, greater than 0 and less than 4. The lithium silicate phase has the formula: Li ２ｚ SiO ２＋ｚ(0<z<2). Preferably, z satisfies the relationship 0<z<1, and more preferably z=1 / 2. Examples of elements other than Li, Si, and O that can be contained in the lithium silicate phase include iron (Fe), chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), molybdenum (Mo), zinc (Zn), and aluminum (Al).

[0060] The carbon phase may be composed of, for example, amorphous carbon with low crystallinity (i.e., amorphous carbon). The amorphous carbon may be, for example, hard carbon, soft carbon, or other.

[0061] The negative electrode current collector may be a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet). Examples of the material for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys.

[0062] [Electrolyte] The electrolyte contains a solvent and a solute dissolved in the solvent. The solute is an electrolyte salt that ionically dissociates in the electrolyte. The solute may include, for example, a lithium salt. Components of the electrolyte other than the solvent and the solute are additives. The electrolyte may contain various additives.

[0063] The solvent may be an aqueous solvent or a non-aqueous solvent. Examples of non-aqueous solvents that can be used include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), and vinylene carbonate (VC). Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of chain carboxylic acid esters include methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP). One type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

[0064] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO4, LiAlCl4, LiB 10 Cl 10 etc.), lithium salts of fluorine-containing acids (LiPF6, LiPF ２ O ２ , LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium salts of fluorine-containing acid imides (LiN(FSO2)2, LiN(CF3SO2)2, LiN(CF3SO2)(CF4F9SO2), LiN(CF5SO2)2, etc.), lithium halides (LiCl, LiBr, LiI, etc.), etc., can be used. One type of lithium salt may be used alone, or two or more types may be used in combination.

[0065] The concentration of the lithium salt in the electrolyte solution may be 1 mol / L or more and 2 mol / L or less, or 1 mol / L or more and 1.5 mol / L or less. By controlling the lithium salt concentration within the above range, an electrolyte solution having excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0066] [Separator] It is desirable to interpose a separator between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulating properties. The separator may be made of a microporous thin film, woven fabric, nonwoven fabric, or the like. The separator is preferably made of polyolefin such as polypropylene or polyethylene.

[0067] An example of the structure of a secondary battery is a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween is housed in an exterior body together with an electrolyte solution. However, this is not limited thereto, and other forms of electrode groups may also be applied. For example, a stacked electrode group in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween may also be used. The shape of the battery is also not limited, and may be, for example, a cylindrical type, a prismatic type, a coin type, a button type, a laminate type, or the like.

[0068] Hereinafter, the structure of a prismatic non-aqueous secondary battery will be described as an example of a secondary battery according to the present disclosure with reference to FIG.

[0069] The battery includes a bottomed, rectangular battery case 4, and an electrode group 1 and an electrolyte (not shown) housed within the battery case 4. The electrode group 1 includes a long, strip-shaped negative electrode, a long, strip-shaped positive electrode, and a separator interposed therebetween. The negative electrode current collector is electrically connected to a negative electrode terminal 6 provided on a sealing plate 5 via a negative electrode lead 3. The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. The positive electrode current collector is electrically connected to the back surface of the sealing plate 5 via a positive electrode lead 2. In other words, the positive electrode is electrically connected to the battery case 4, which also serves as the positive electrode terminal. The periphery of the sealing plate 5 fits into the open end of the battery case 4, and the fitting is laser-welded. The sealing plate 5 has an injection hole for the nonaqueous electrolyte, which is closed with a seal plug 8 after injection.

[0070] Hereinafter, the present disclosure will be specifically described based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0071] Example 1 [Fabrication of Positive Electrode] As the first positive electrode active material particles, lithium-cobalt-aluminum composite oxide (LiNi 0.９１ Co 0.０５ Al ０．０４ The particle size L of the first positive electrode active material particles, measured as the D80 diameter in the volume-based particle size distribution by a laser diffraction scattering method, was 6 μm.

[0072] The second and third positive electrode active material particles are made of lithium-nickel-manganese composite oxide (LiNi) having different average particle diameters. 0.8 Mn 0.2 The particle sizes R and r of the second and third positive electrode active material particles, measured as the D80 diameter in the volume-based particle size distribution by a laser diffraction scattering method, were 15 μm and 1 μm, respectively.

[0073] A first positive electrode slurry was prepared by mixing 98 parts by mass of the first positive electrode active material particles, 1 part by mass of acetylene black (AB), 1 part by mass of polyvinylidene fluoride (PVDF), and an appropriate amount of N-methyl-2-pyrrolidone (NMP).

[0074] A second positive electrode slurry was prepared by mixing 98 parts by mass of a positive electrode active material obtained by mixing second positive electrode active material particles and third positive electrode active material particles in a mass ratio of second positive electrode active material particles:third positive electrode active material particles = 7:3, 1 part by mass of acetylene black (AB), 1 part by mass of polyvinylidene fluoride (PVDF), and an appropriate amount of NMP.

[0075] The first positive electrode slurry was applied to the surface of an aluminum foil serving as a positive electrode current collector, the coating was dried, and then the second positive electrode slurry was applied to cover the coating of the first positive electrode slurry. The coating was then dried and rolled to form a 100 μm-thick positive electrode mixture layer on the aluminum foil. In the positive electrode mixture layer, the thickness of the first layer containing the first positive electrode active material particles and the thickness of the second layer containing the second and third positive electrode active material particles were adjusted so that the ratio of the total mass of the second positive electrode active material particles and the third positive electrode active material particles to the mass of the first positive electrode active material particles was 8:2.

[0076] Using a cross-section polisher (CP), a cross section was formed by simultaneously cutting the positive electrode mixture layer and the positive electrode current collector in the thickness direction, and an SEM image of the cross section was taken. The maximum diameters (maximum circle-equivalent diameters) of the first, second, and third positive electrode active material particles were determined by the method described above. As a result, L, R, and r determined as the maximum circle-equivalent diameters were approximately equal to L, R, and r determined as the D80 diameter in the volume-based particle size distribution, respectively.

[0077] The positive electrode was cut into a predetermined shape to obtain a positive electrode for evaluation. The positive electrode had a 20 mm × 20 mm region to function as a positive electrode and a 5 mm × 5 mm region to connect to the tab lead. The positive electrode mixture layer formed on the connection region was then scraped off to expose the positive electrode current collector. The exposed portion of the positive electrode current collector was then connected to the positive electrode tab lead, and a predetermined region around the periphery of the positive electrode tab lead was covered with an insulating tab film.

[0078] [Fabrication of Negative Electrode] A lithium metal foil (thickness: 300 μm) was attached to one side of an electrolytic copper foil serving as a negative electrode current collector to fabricate a negative electrode.

[0079] The negative electrode was cut into the same shape as the positive electrode to obtain a negative electrode for evaluation. The lithium metal foil formed on the connection area was peeled off in the same way as the positive electrode to expose the negative electrode current collector. The exposed portion of the negative electrode current collector was then connected to a negative electrode tab lead, and a predetermined area around the periphery of the negative electrode tab lead was covered with an insulating tab film, just like the positive electrode.

[0080] [Preparation of Electrolyte Solution] An electrolyte solution was prepared by adding LiPF as a lithium salt to a mixed solvent containing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 20:5:75. The concentration of LiPF in the electrolyte solution was 1.3 mol / L.

[0081] [Preparation of Secondary Battery] A battery for evaluation was prepared using a positive electrode and a negative electrode for evaluation. First, the positive electrode and the negative electrode were opposed to each other with a separator interposed between them so that the positive electrode mixture layer and the negative electrode mixture layer overlapped to obtain an electrode plate assembly. Next, an Al laminate film (thickness 100 μm) cut into a 60 × 90 mm rectangle was folded in half, and the end of the 60 mm long side was heat-sealed at 230 ° C to form a 60 × 45 mm cylindrical shape. Then, the prepared electrode plate assembly was placed in the cylinder, and the end face of the Al laminate film was aligned with the insulating tab film of each tab lead and heat-sealed at 230 ° C. Next, nonaqueous electrolyte was poured 0.3 cm from the short side of the Al laminate film that was not heat-sealed. ３ After the injection, the mixture was left standing for 5 minutes under a reduced pressure of 0.06 MPa to allow the electrolyte to penetrate into each mixture layer. Finally, the end face of the Al laminate film on the injected side was heat-sealed at 230°C to prepare a battery A1 for evaluation. The evaluation cell was prepared in a dry environment with a dew point of -50°C or lower.

[0082] Comparative Example 1 In the preparation of a positive electrode, only the second positive electrode slurry of Example 1 was applied to the surface of an aluminum foil serving as a positive electrode current collector, and the other procedures were carried out in the same manner as in Example 1 to obtain a positive electrode having the same theoretical capacity as in Example 1 and a positive electrode mixture layer with a thickness of 100 μm. Using this positive electrode, a battery B1 for evaluation was prepared in the same manner as in Example 1.

[0083] <Comparative Example 2> In the preparation of a positive electrode, 98 parts by mass of the positive electrode active material obtained by mixing the first to third positive electrode active material particles of Example 1 in a mass ratio of first positive electrode active material particles: second positive electrode active material particles: third positive electrode active material particles = 20:56:24 was mixed with 1 part by mass of acetylene black (AB), 1 part by mass of polyvinylidene fluoride (PVDF), and an appropriate amount of NMP to prepare a positive electrode slurry. Only the positive electrode slurry was applied to the surface of an aluminum foil positive electrode current collector, and the rest was treated in the same manner to obtain a positive electrode having a positive electrode mixture layer with a thickness of 100 μm. In this positive electrode, the content of the first to third positive electrode active material particles in the positive electrode mixture layer was the same as that of the positive electrode of Example 1. However, the positive electrode mixture layer did not have a first layer and a second layer, and the first to third positive electrode active material particles were dispersed within the positive electrode mixture layer.

[0084] Using this positive electrode, a battery B2 for evaluation was fabricated in the same manner as in Example 1.

[0085] [Evaluation 1: Charge / Discharge Curve] The completed battery was clamped between a pair of 80 x 80 cm stainless steel clamps (2 mm thick) and pressurized to 0.2 MPa. Each battery was placed in a 25°C environment and charged / discharged. Charging was performed by applying a constant current of 0.7 C until the voltage reached 4.3 V, and then maintaining the voltage at 4.3 V until the current value fell to 0.07 C or less. Discharging was performed at a constant current of 0.15 C until the voltage reached 2.5 V. The relationship between the total amount of current flowing and the battery voltage (charge / discharge curve) was then determined. The rest period between charging and discharging was 20 minutes, and 10 charge / discharge cycles were repeated under the above charge / discharge conditions in a 25°C environment.

[0086] [Evaluation 2: Capacity Retention Rate] The battery was placed in an environment of 25° C., and the same charge / discharge cycles as in Evaluation 1 were carried out except that the charge voltage was set to 4.5V.

[0087] The rest period between charge and discharge was 20 minutes, and 10 cycles of charge and discharge were repeated under the above charge and discharge conditions in an environment of 25°C. ０ Discharge capacity C ｎ Calculate Cn / C ０ ×100 was evaluated as the capacity retention rate.

[0088] Figure 3A shows the measurement results of the charge / discharge curve of Battery A1. Figure 3B shows the measurement results of the charge / discharge curve of Battery B1. Both Battery A1 and Battery B1 show a tendency for the voltage during charging to increase and the discharge capacity to decrease with repeated charge / discharge cycles. However, in Figure 3A, the increase in charge voltage with repeated charge / discharge cycles is suppressed compared to Figure 3B, and polarization is reduced. Furthermore, the decrease in discharge capacity with repeated charge / discharge cycles is suppressed.

[0089] Figure 4 shows the change in capacity retention rate for batteries A1, B1, and B2 with each charge-discharge cycle. In Figure 4, the batteries were charged at a high voltage of 4.5 V until they reached an overcharged state, creating an environment in which Mn in the lithium-nickel-manganese composite oxide is likely to dissolve. However, even under these high-voltage charging conditions, battery A1 exhibits a reduced decrease in discharge capacity with repeated charge-discharge cycles compared to batteries B1 and B2.

[0090] Examples 2 to 6, Comparative Example 3 In the preparation of the positive electrodes, the particle sizes of the first to third positive electrode active material particles were changed as shown in Table 1. Otherwise, similarly to Example 1, evaluation batteries A2 to A6 and B3 were prepared, and the discharge rate characteristics were evaluated by the method shown below.

[0091] [Evaluation 3: Discharge Rate Characteristics] A pair of identical batteries was prepared, and each battery was placed in an environment of 25°C and subjected to constant current charging at a current of 0.15 C until the voltage reached 4.3 V, and then constant voltage charging at a constant voltage of 4.3 V until the current reached 0.015 C.

[0092] Thereafter, one of the batteries was discharged at a constant current of 1 C until the voltage reached 2.5 V, and the discharge capacity C １ The other battery was discharged at a constant current of 0.1 C until the voltage reached 2.5 V, and the discharge capacity C １０ was calculated. (C １ / C １０ ) × 100 was calculated and evaluated as the discharge rate characteristic.

[0093] The evaluation results of the discharge rate characteristics of Batteries A1 to A6 and B3, along with the particle sizes of the first to third positive electrode active material particles, are shown in Table 1. Batteries A1 to A5, which satisfied R>L>r and r>0.155L, were able to maintain high discharge rate characteristics.

[0094] The charge-discharge curve of Battery A6 is shown in Figure 5. Because Battery A6 satisfied the relationship R > L > r, the increase in charge voltage with repeated charge-discharge cycles was suppressed, and the decrease in discharge capacity with repeated charge-discharge cycles was also suppressed, although the battery performance was inferior to that of Battery A1. However, because Battery A6 did not satisfy the relationship r > 0.155 L, the discharge rate characteristics were inferior to those of Batteries A1 to A5.

[0095] Battery B3 did not satisfy R>L>r, and therefore was unable to suppress the increase in charging voltage and the decrease in discharge capacity that occurred with repeated charge-discharge cycles, and its discharge rate characteristics were also inferior to those of batteries A1 to A6.

[0096]

[0097] The secondary battery according to the present disclosure can provide a high-capacity secondary battery that is advantageous in terms of improving charge-discharge characteristics. The secondary battery according to the present disclosure is useful as a main power source for mobile communication devices, electric vehicles, hybrid vehicles, portable electronic devices, etc.

[0098] While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention.

[0099] 1: electrode group, 2: positive electrode lead, 3: negative electrode lead, 4: battery case, 5: sealing plate, 6: negative electrode terminal, 7: gasket, 8: sealing plug, 10: positive electrode, 11: positive electrode current collector, 12: positive electrode mixture layer, 12A: first layer, 12B: second layer, P1: first positive electrode active material particle, P2: second positive electrode active material particle, P3: third positive electrode active material particle

Claims

1. A positive electrode current collector and a positive electrode mixture layer provided on the surface of the positive electrode current collector, wherein the positive electrode current collector contains Al, the positive electrode mixture layer includes a first layer in contact with the positive electrode current collector and a second layer in contact with the first layer, the first layer contains first positive electrode active material particles having a particle size L, the second layer contains second positive electrode active material particles having a particle size R, at least at the interface between the first layer and the second layer, third positive electrode active material particles having a particle size r are included, the first positive electrode active material particles contain a first lithium transition metal composite oxide, and the proportion of Co in the metal elements other than Li contained in the first lithium transition metal composite oxide is 2 atomic% or more, the second positive electrode active material particles contain a second lithium transition metal composite oxide, and the second lithium transition metal composite oxide either does not contain Co or the proportion of Co in the metal elements other than Li contained in the second lithium transition metal composite oxide is less than 2 atomic%, the third positive electrode active material particles contain a third lithium transition metal composite oxide, and the third lithium transition metal composite oxide either does not contain Co or the proportion of Co in the metal elements other than Li contained in the third lithium transition metal composite oxide is less than 2 atomic%, A positive electrode for a secondary battery, satisfying R > L > r.

2. The positive electrode for a secondary battery according to Claim 1, satisfying r > 0.155L.

3. The particle size R of the second positive electrode active material particles is in the range of 10 to 30 μm, The particle size r of the third positive electrode active material particles is in the range of 1 to 5 μm, The positive electrode for a secondary battery according to Claim 1.

4. The positive electrode for a secondary battery according to Claim 1, wherein no third positive electrode active material particles are present at the interface between the positive electrode current collector and the first layer.

5. The positive electrode for a secondary battery according to Claim 1, wherein the second layer contains the third positive electrode active material particles.

6. The positive electrode for a secondary battery according to Claim 1, wherein the first lithium transition metal composite oxide contains Ni, Co, and Al.

7. The first lithium transition metal composite oxide is Li α Ni １－ｘ―ｙ Co ｘ Al ｙ O2 (where 0.95 ≤ α ≤ 1.05, 0.02 ≤ x ≤ 0.1, 0.02 < x + y < 1), which is a lithium-nickel-cobalt-aluminum composite oxide, and the positive electrode for a secondary battery according to claim 6.

8. The second lithium transition metal composite oxide and the third lithium transition metal composite oxide each contain a lithium-nickel-manganese composite oxide containing Ni and Mn and having a proportion of Ni and Mn in the metal elements other than Li of 98 atomic% or more, The positive electrode for a secondary battery according to Claim 1.

9. The lithium-nickel-manganese composite oxide is Li α Ni １－ｘ Mn ｘ O ２ (where 0.95 ≦ α ≦ 1.05, 0 < x ≦ 0.2), the positive electrode for a secondary battery according to claim 8.

10. A positive electrode for a secondary battery according to any one of Claims 1 to 9, and A secondary battery having a separator, a negative electrode facing the positive electrode for the secondary battery via the separator, and an electrolytic solution.